This invention relates to a Group 4 metal complex, to a catalyst composition and to a process for polymerizing addition polymerizable unsaturated monomers, especially olefins. In particular, the invention is directed to certain Group 4 metal complexes, to catalyst compositions comprising the same, and to addition polymerization processes using the same.
Advances in polymerization and catalysis have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts the choice of polymerization-type (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Recently, several new disclosures related to metal complexes based on polyvalent metal-centered, heteroaryl donor ligands have published. Among these are U.S. Pat. Nos. 6,103,657, 6,906,160, 6,919,407, 6,927,256, US-A-2002/0142912, US-A-2004/0220050, US-A-2004/0005984, WO 2000/020377, and WO 2002/038628.
Regardless of the technological advances in the polyolefin industry afforded by this new class of catalyst, common problems, as well as new challenges associated with process operability, exist. For example, known Group 4 metal complexes based on donor ligands can produce extremely high molecular weight polymers, which in a solution polymerization can result in highly viscous reaction mixtures leading to high stirring forces and energy requirements. To decrease viscosity of the reaction mixture, higher polymerization temperatures may be employed. Disadvantageously however, higher reaction temperatures normally result in a reduction in polymer tacticity or crystallinity, especially when normally tactic, especially isotactic polymers, are produced. Alternatively, it is known to add a chain transfer agent such as hydrogen to the reactor to produce lower molecular weight polymers, thereby incidentally reducing polymer viscosity and ultimately, reaction mixture viscosity. Besides adding additional cost and complexity to the process, the solubility of standard chain transfer agents generally decreases at higher reaction temperatures thereby limiting their effectiveness. Additionally, the resulting polymers also tend to have broadened molecular weight distributions, rendering the product unsuited for some desired end uses.
Thus, it would be advantageous to provide a solution polymerization process for the polymerization of olefin monomers employing specific metal complexes based on donor ligands that are capable of operation at high temperatures and efficiencies and adapted to produce polymers having a desired molecular weight or melt flow while retaining the ability to control the molecular weight distribution of the resulting product. Moreover, it would be advantageous to provide a solution polymerization process for the polymerization of olefin monomers that is capable of operation at high temperatures and efficiencies and adapted to produce polymers having low molecular weights while using reduced quantities of chain transfer agent, especially hydrogen. Finally, it would be advantageous to provide a solution polymerization process for preparing tactic polymers, especially isotactic homopolymers and copolymers comprising propylene and/or a C4-20 olefin that is capable of operation at high temperatures and adapted to produce polymers having a relatively low molecular weight while maintaining a relatively high tacticity or crystallinity, particularly one that does not require the use of excessive quantities of hydrogen or other chain transfer agent.